Variable attenuator with linear drive



Dec. 11, 1951 s. A. JOHNSON VARIABLE ATTENUATOR WITH LINEAR DRIVE Filed Dec;

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Patented Dec. 11, 1951 UNITED STATES PATENT OFFICE VARIABLE ATTENUA'IOR WITH LINEAR DRIVE Stanley A. Johnson, New York, N. Y., assignor to Polytechnic Institute of Brooklyn, Brooklyn, N. Y., a corporation of New York Application December 4, 1946, Serial No. 714,117

9 Claims. (Cl. 17844) 'This invention relates to variable attenuators for wave guides, and it is especially concerned with attenuators of the guillotine type in which a loss-producing plate-like element is inserted into the wave guide through a longitudinal slot In wave guide attenuators of this type, the

amount of attenuation normally increases exponentially with the amount of plate insertion This is an undesirable characteristic, since the attenuation scale at the larger attenuation values becomes crowded.

One object of the present invention is to devise a drive arrangement for the attenuator plate whereby the attenuation scale is spread out and rendered substantially linear over its major working portion.

A further object of the invention is to devise a novel arrangement for mounting the attenuator plate for movementinto and out of the slot in the wave guide.

A further object is to provide means for preventing loss of wave energy through the slot in the wave guide.

The preferred form of the invention is illustrated in the accompanying drawing in which Figure 1 is a plan view of the variable attenu ator;

Figure 2 is a side elevational view of Figure l with one cover plate removed and certain parts shown in section;

Figure 3 is a graph of two curves illustrating the operating characteristic of the attenuator; and

Figure 4 is a graph of curves illustrating the effect of the chokes provided adjacent the attenuator slot.

Referring to Figures 1 and 2 of the drawing, I

indicates a short section of rectangular wave coupling element isfree to rotate about the wave guide I and servesto clamp a circular head Ia 'rigidly secured to the end of guide I against the end of a male coupling element mounted on the adjacent section of wave guide to be coupled to the attenuator. The male coupling element 2 is '20 which extend into similarly located slots formed in the peripheral edge of the head la of the female couplin on the adjacent wave guide section. These lugs maintain proper alignment between the coupled wave guide sections.

As shown in Figure 2, the mounting frame and casing for the attenuator is formed of an inverted U-shaped frame 4 positioned on the top broad face of the wave guide I, and a bar-shaped member arranged below the wave guide. Members 4 and 5 are of the same width as the guide I, and a pair-of cover plates 6 and I are secured on opposite sides of these frame members 4 and 5 to form an enclosed metallic housing in the space within the member 4 above the wave guide.

Mounted within the attenuator housing is a movable block 8 which carries an attenuator plate 9, the lower edge of which is rounded as shown at 9:1. This attenuator plate is preferably formed of a relatively thin plate of dielectric material such as glass, and carries on one face thereof a thin metallic film which will produce an energy loss without introducing substantial resonant effects in the wave guide. The metallic film is preferably deposited by thermal evaporation in accordance with the process disclosed in copending application Serial No. 699,546, filed September 26, 1946. It will be obvious, however, that any other form of loss-producing platelike element may be employed instead of that just described. The attenuator plate 9 is pref- "erably mounted in the block 8 by means of a slot formed in the block and the upper edge portion of the plate is cemented into the slot.

The block 8 is mounted within the attenuator casing to have translational movement having both longitudinal and. vertical components of motion simultaneously, and this is accomplished by a pair of pivoted links Iila, Illb and a spring I]. The two links Ilia and Illb are short bars having V-shaped grooves formed in their ends which engage round roller bearin elements I9 and I0" mounted respectively on the frame member 4 and on the block 3. The tension spring II is connected between the block 8 and the frame 4, the spring being arranged to urge the block 8 in a direction such that the rollers III" maintain constant contact with the lower ends of links Mia and I db, and the upper ends of these links maintain constant contact with the rollers III.

With this type of mounting, the links Illa will swing as if they were pivoted about the centers of pins If! and I0", and the block 8 will remain parallel with the wave guide I in all positions.

Furthermore, the action of spring II tends to move the block 8 to the right in all positions of the block against the end of an adjusting screw I2 having threaded engagement with the right end of frame member 4 and having an adjusting knob I3.

At the other end of the attenuator casing is mounted a micrometer I4 which may be calibrated in units of attenuation such as decibels. The micrometer I4 is preferably mounted to respond to the longitudinal movement of the block 8, and for this purpose the mounting stem I la extends through a hole in the left leg of the frame member 4 and is clamped in position by a set screw I5. The operating plunger I4b of the micrometer bears against the left end of the block 8.

With the construction as illustrated in Figure 2, the block 8 is constantly urged into contact.

with the end of adjusting screw I2, and the block is shown in Figure 2 in the position of maximum attenuation, or in the position of maximum insertion of the plate 9 into the wave guide. As the screw I2 is advanced into the attenuator casing, the block 8 is moved to the left and, due to the pivotal mounting of the links Ida and I012, the block will also have a vertical component of movement resulting in the outward movement of the attenuator plate from the wave guide. Starting from the position of maximum attenuation as shown in Figure 2, the vertical component of movement is small by comparison with the horizontal component, but the ratio of vertical .to horizontal movement increases as the screw I2 is advanced further into the attenuator casing. By arranging the micrometer I4 to respond to the horizontal movement alone, a near-linear relation is obtained between the movement of the screw I2 and the amount of attenuation introduced into the wave guide. Curve A in Figure 3 illustrates the relation between the horizontal movement of the block 8 (dial readings of micrometer I l) and the attenuation in decibels. As will be seen, this curve is a substantially straight line over the major portion of its length. Curve B in Figure 3 is a typical curve showing the standing wave ratio for different positions of attenuator adjustment.

From the foregoing it will be seen that the attenuator plate is mounted for translational movement within the slot in the wave guide in such manner that the plate has both horizontal and vertical components of movement simultaneously. The arrangement is such that the attenuatorplate is constrained to move in the arc of a circle defined by a pivoted arm or link which in the position of maximum attenuation is substantially at right angles to the wave guide. Two parallel links are used inorder to maintain the block 8 parallel with the wave guide I in, all positions, but it is obvious that only one pivoted link is necessary if other means is employed to maintain the block parallel to the wave guide.

It has been found that the portion of the attenuator plate 9 extending outside of the wave guide acts as an antenna for launching waves into the attenuator casing from the wave guide, and this shunts energy from the wave guide, thus limiting the maximum attenuation obtainable to the attenuation of the shunt path. For the purposeoflpreventingthis shunting of energy from the wave guide, a pair of magnetic chokes are mounted on each side of the attenuator plate adjacent the slot in the wave guide. These chokes are formed of bars I6 of suitable magnetic material arranged on opposite sides of plate 9 and mounted upon a slotted plate I! which is soldered or otherwise secured to the top face of the wave guide I. Plate II serves to prevent longitudinal movement of the attenuator casing. Preferably, these choke bars are formed of Polyiron consisting of fine iron particles held together by a suitable binder.

The curves in Figure 4 illustrate the operation of the wave attenuator with and without chokes. Curve C shows the operation without the magnetic chokes and curve D shows the operation with the chokes. As will be seen, greater attenuation is obtained with the chokes and the curve is more linear.

For the purpose of illustration, the dimensions will begiven of a variable attenuator suitable for use in the wave-band of 1.225 cm. to 1.275 cm. with a maximum attenuation of 40 decibels. The wave guide for this band measures A2 inch by inch by 0.040 inch (Wall thickness). The attenuation plate 9 is formed of Pyrex glass 0.038 inch thick, and the metal film on the plate has a resistance of ohms per square. The plate has a length of 2 inches and a maximum width of 0.6 inch. The radius of curvature of the lower edge of the plate is 2 inches. The parallel link mounting for the block 8 permitted horizontal travel of 0.200 inch with a corresponding vertical travel of 0.170 inch.

The slot through which the attenuator plate enters the wave guide is 1 3' inch wide, and the Polyiron strips were spaced 0.012 inch from the respective sides of the plate. The insertion loss of the complete unit is less than 0.3 db and the input VSWR is less than 1.12 for all attenuation settings within the band. The attenuation change between the band edge frequencies is below 1.5 db.

Whilev a specific example has been given for a particular band of waves, it is obvious that the invention may be applied to other wave bands if desired.

The rounded lower edge of plate 9 serves to match the input impedance of the attenuator with the characteristic impedance of the wave guide, thus preventing noticeable wave reflection.

The parallel links Illa and I0b serve to guide the movement of theplate 9 in the wave guide along the arc of a circle, with the result that the rate of insertion of the plate into the guide varies over the range of movement of the plate. The ratio of the vertical component of movement to the longitudinal component decreases as the insertion increases, so that there is substantially a straight. line relation between the attenuation and the longitudinal component of movement overthe major portion of the range. This establishes substantially a straight line relation be- .tween the attenuation and the dial readings of micrometer I 4 or the movement of adjusting screw I2.

The term. translational movement has reference to motion in which all t e points of the moving bodv have at any instant the same velocitv and direction of motion.

We claim:

1. An attenuator for a wave guide adapted to convey electromagnetic wave energy having a slot formed longitudinally therein, a loss-producing. plate-like element mounted for movement in saidslot, and means forguiding said plate element for movement in said slot comprising a pair of .parallellinks pivotedat one end toa fixed support spacedfrom. said wave guide on the slotted side thereof and pivotally connected at the other end to spaced points on said plate element.

2. An attenuator according to claim 1 and including an indicator and means for actuating said indicator by the component of movement of said plate element longitudinally of said wave guide.

3. An attenuator according to claim 1 wherein said parallel links are enclosed within a metallic housing surrounding said slot, and including a pair of choke elements mounted on opposite sides of said slot closely adjacent the faces of said plate element.

4. An attenuator according to claim 1 and including a spring element acting on said platelike element tending to move said plate-like element in one direction, and an adjusting screw mounted with its axis parallel with the axis of said waveguide and arranged to move said platelike element in the opposite direction.

5. An attenuator for a waveguide having a slot formed longitudinally in one wall thereof comprising a loss-producing, plate-like element, means for mounting said element for movement into said slot comprising an elongated bar to which said plate element is attached, a pair of parallel links pivotally attached to said bar at spaced points thereon and having pivotal connections at their opposite ends with a fixed support at spaced points thereon, said bar being parallel with said waveguide, spring means interposed between said bar and said fixed support and tending normally to move said bar longitudinally in one direction, and an adjusting screw having threaded engagement with said fixed support with its axis parallel with said waveguide and arranged to impart longitudinal movement to said bar in the opposite direction.

6. An attenuator according to claim 5 and including an indicator mounted to respond to the longitudinal movement of said bar.

7. An attenuator for a wave guide having a slot formed longitudinally therein, a loss-producing plate-like element positioned within said slot, a movable adjusting member mounted for movement in a direction parallel with the axis of said wave guide and arranged to eiTect longitudinal movement of said plate-like element in said slot, and means controlled by the longitudinal movement of said plate-like element to impart movement of said plate element transversely of said wave guide and thereby to vary the extent of insertion of said plate-like element into said slot, the transverse component of movement being so related to the longitudinal component as to establish substantially a linear relation between the longitudinal movement of said adjusting member and the attenuation produced by said plate-like element.

8. An attenuator for a waveguide adapted to convey electromagnetic wave energy comprising, a section of waveguide having a slot formed longitudinally in one wall thereof, a loss-producing plate-like element mounted for movement through said slot, means for guiding said plate element through said slot with translational movement having a component of movement parallel with the longitudinal axis of said section and a component of movement directed transversely of said section, the ratio of said longitudinal component to said transverse component increasing progressively as said plate moves from a position of minimum insertion to a position of maximum insertion into said section, an attenuation indicator, and means for actuating said indicator in accordance with the longitudinal component of movement of said plate element.

9. An attenuator for a waveguide adapted to convey electromagnetic wave energy comprising, a section of waveguide having a slot formed longitudinally in one wall thereof, a loss-producing plate-like element mounted for movement through said slot, and means for guiding said plate into said slot with translational movement wherein all points of said plate travel along circular arcs of equal radii and having their centers of movement spaced from said waveguide on the slotted side thereof.

STANLEY A. JOHNSON.

REFERENCES CITED The followin references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,427,039 Trane Aug. 22, 1922 2,306,282 Samuel Dec. 22, 1942 2,427,107 Landon Sept. 9, 1947 2,431,941 Kihn Dec. 2, 1947 2,465,719 Fernsler Mar. 29, 1949 2,479,650 Tiley Aug. 23, 1949 2,491,644 Carlson Dec. 20, 1949 FOREIGN PATENTS Number Country Date 576,145 Great Britain Mar. 20, 1946 OTHER REFERENCES A. I. E. E. Technical Paper 46-40January 1946-The article Techniques and Facilities For Microwave Radar Testing.

Proceedings of the I. R. E. October, 1946 pages 782-785.

Electrical Engineering, Published by American Institute of Electrical Engineers, May 1946. Pages 274-290. 

